ANNOTATIONS AND ADDITIONS. 311 



( 14 ) p. 238. "If the height of the aerial ocean and its pressure have 

 not always been the same." 



The pressure of the atmosphere has a decided influence on the 

 form and life of plants. From the abundance and importance of 

 their leafy organs provided with porous openings, plants live prin- 

 cipally in and through their surfaces ; and hence their dependence 

 on the surrounding medium. Animals are dependent rather on in- 

 ternal impulses and stimuli ; they originate and maintain their own 

 temperature, and, by means of muscular movement, their own 

 electric currents, and the chemical vital processes which depend on 

 and react upon those currents. A species of skin-respiration is an 

 active and important vital function in plants, and this respiration, 

 in so far as it consists in evaporation, inhalation, and exhalation of 

 fluids, is dependent on the pressure of atmosphere. Therefore it is 

 that alpine plants are more aromatic, and are hairy and covered with 

 numerous pores. (See my work, iiber die gereizte Muskel- und Ner- 

 venfaser, bd. ii. s. 142145.) For, according to Zoonomic experience, 

 organs become more abundant and more perfect in proportion to the 

 facility with which the conditions necessary for the exercise of their 

 functions are fulfilled as I have elsewhere shown. In alpine 

 plants, the disturbance of their skin-respiration, occasioned by in- 

 creased atmospheric pressure, makes it very difficult for such plants 

 to flourish in the low grounds. 



The question, whether the mean pressure of the aerial ocean 

 which surrounds our globe has always been the same, is quite un- 

 decided : we do not even know accurately whether the mean height 

 of the barometer has continued the same at the same place for a 

 century past. According to Poleni's and Toaldo's observations, the 

 pressure would have seemed to vary. The correctness of these ob- 

 servations has long been doubted, but the recent researches of Car- 

 lini render it almost probable that the mean height of the barome- 

 ter is diminishing in Milan. Perhaps the phenomenon is a very 

 local one, and dependent on variations in descending atmospheric 

 currents. 



